As a method of semiconductor mounting capable of achieving higher productivity, a bare chip mounting has been developed, wherein a semiconductor element having a bump and a substrate are joined and the space between them is filled with an underfill material by immersion and the like. In such bare chip mounting, an electrode on a semiconductor element is formed into a convex shape (bump) using, for example, a soldering paste, and this bump is metal joined with a circuit on a substrate and the gap between the semiconductor element and the substrate (other than the bump) is filled with an underfill material to form a bump junction.
A semiconductor element mounted by this method, however, easily suffers from the stress produced by the difference in the expansion coefficients of the substrate and the semiconductor element, often resulting in a defective bump junction. In this event, a greater distance between the semiconductor element and the substrate during the above-mentioned junction reduces the stress produced by the difference in the expansion coefficients of the substrate and the semiconductor element. However, a greater distance between the semiconductor element and the substrate requires a greater diameter of the bump to match the distance, thus making a fine-pitch connection difficult to achieve.
Because the underfill material comprises a liquid resin for adhesion, the fluidity of the adhesive material needs control. Thus, the development of an adhesive material capable of simple adhesion is desired to replace the liquid resin.
As a different method for semiconductor mounting, a method is known, wherein a semiconductor element and a circuit board are adhered using an anisotropic conductive film to achieve conduction. When compared with the above-mentioned method for forming a bump junction, however, the connection resistance becomes higher. Therefore, when this method is used for a high speed semiconductor device, many problems occur, such as heat generation in a semiconductor element, noise signal during operation and the like.
To obviate these defects, the present inventors conceived an idea of adhering, to a semiconductor wafer, a porous adhesive sheet generally used for adhering a filter and the like to secure gas permeation, filling part of the through holes in the porous adhesive sheet with a soldering paste to electrically connect the semiconductor element and one side of the porous adhesive sheet, forming a bump on one side of the porous adhesive sheet to connect the circuit side of the substrate, thereby to improve connection reliability with the circuit side. As a porous adhesive sheet usable for such semiconductor mounting, one having through holes of a regular shape, which are hardly closed in an adhesion state, is preferable.
When a production method comprising forming a number of fine through holes in a formed adhesive sheet to give a porous adhesive sheet is employed, a resin, which is a sheet material, flows during adhesion and fills the through holes, failing the conduction.
When a production method of a porous adhesive sheet is employed, which comprises extending a formed organic film to give an adhesive sheet and forming a number of fine through holes, moreover, respective through holes thus formed do not have a regular shape. When the organic film itself has adhesiveness that allows adhesion by heating and/or pressurization, the opening of some of the through holes is easily closed up during adhesion, and the opening ratio of the porous adhesive sheet drastically decreases before and after the adhesion. With such production method, formation of through holes such that the porous adhesive sheet has such opening ratio as to achieve the object is difficult.
The present inventors assumed the following processes for forming a number of fine through holes in an organic film. These methods provide through holes having a regular shape, but each has the following problems.
(1) Drilling
Due to low productivity, unsuitable for the production of a porous adhesive sheet having a number of fine through holes.
(2) Punching
Incapable of forming fine through holes, unsuitable for production of the above-mentioned porous adhesive sheet.
(3) Laser Beam Machining
A porous adhesive sheet wherein each through hole has an about trapezoid shape is produced, due to which the opening ratio (area ratio of opening of through holes to the entire porous adhesive sheet) of one surface of one side differs greatly from that of one surface of the other side upon formation of the through holes. In this porous adhesive sheet, the ratio of the minimum area Smin to the maximum area Smax, Smin/Smax (%), of the sections in the diameter direction from one opening to the other opening of a through hole is 40%-80%, wherein the area permitting adhesion on the side having a greater opening ratio becomes smaller than that on the side having a smaller opening ratio. This has a consequence that the side having a greater opening ratio of the adhesive sheet fails to have adhesiveness permitting its adhesion to an adhesion target.
(4) Photoprocessing
As in the case of laser beam machining, each through hole has an about trapezoid shape. In this porous adhesive sheet, the ratio of the minimum area Smin to the maximum area Smax, Smin/Smax (%), of the sections in the diameter direction from one opening to the other opening of a through hole is 40%-80%. As a result, a porous adhesive sheet wherein the opening ratio differs greatly between one surface of one side and one surface of the other side is unpreferably produced.
The present invention aims at solving the above-mentioned problems and provides the following.    (1) A porous adhesive sheet suitably used also in the field of electronic materials.    (2) A suitable production method of the adhesive sheet of the above-mentioned (1) (e.g., a porous adhesive sheet having through holes having a regular shape and difficult to close in an adhesion state.    (3) A semiconductor wafer with a porous adhesive sheet, which is suitable for bare chip mounting, and a suitable production method thereof.